The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency, for example in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching element such as a power MOSFET at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted using a feedback signal to convert an input voltage to a desired output voltage. An SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
It is desirable to increase the accuracy with which current in a SMPS can be determined for a number of reasons. For example, the emergence of ever more advanced and computationally intensive signal and communication processing algorithms has fuelled the need for new low-voltage CMOS technology for their implementation. This puts new challenging requirements on the power supply, such as tighter voltage tolerance bands and the ability to provide increased current levels. In order to meet these requirements, it is necessary to improve various aspects of the SMPS's operation (e.g. current feedback control, detection of continuous and discontinuous conduction modes, current protection and system identification) by increasing the accuracy of the current measurement upon which these rely. Accurate current measurement also enables accurate diode emulation in synchronous rectified power converters, thus improving their low load efficiency.
Known current estimation methods employed in an SMPS in the form of a switched mode DC/DC power supply will now be described with reference to FIGS. 1 and 5.
FIG. 1 is a simplified circuit diagram of a switched mode DC/DC power supply 10 which converts an input voltage Vin to a desired output voltage Vout. The power supply 10 comprises an inductor 20, a capacitor 30, a diode 40, a power transistor 50 and a pulse-width modulating (PWM) controller 60. The PWM controller applies voltage pulses 70 at an appropriate frequency (e.g. 30 kHz) to the gate of the power transistor 50. The PWM controller regulates the output voltage Vout by adjusting the duty cycle D of the pulses (defined by D=TON/Ts, where TON is the duration of a pulse and Ts is the switch period) on the basis of a feedback signal which is obtained from a measurement of the current in the inductor. In the example of FIG. 1, the current is measured using resistor 80.
In this arrangement, the current (i) in the inductor 20 varies with time (t) in a generally saw-tooth manner as shown in FIG. 5, increasing from a minimum value Imin to a maximum value Tmax during a period DTs when the transistor is switched ON, before decreasing to Imin during a period (1−D)Ts when the transistor is switched OFF. The PWM controller 60 repeatedly measures the current a number of times during a switch period Ts and calculates a current value using samples obtained during the ON-period DTs or OFF-period (1−D)Ts. However, the switching of the transistor can cause transients which introduce errors in the current values measured shortly after a transition from an ON-period to the following OFF-period or from an OFF-period to the next ON-period. For this reason, it is preferable to disregard the samples obtained in a blanking period TB immediately following a transition when performing a current calculation. In the present example, the calculation is based on a number of measured current sample values which are obtained over a time (1−D)Ts−TB during the OFF-period.
Having obtained the current samples, it is then necessary to process them to calculate an overall current value. A number of known techniques exist for doing this. For example, the use of the statistically robust median value of sampled current values for current protection is described in “ZL2005 Current Protection and Measurement” (Zilkerlabs Application Note AN15, www.zilkerlabs.com), where a digital filter is used for increasing the accuracy of current monitoring over the PMBus. However, this method introduces an offset error that varies with the blanking time and the duty cycle. Hence, the accuracy and latency of this method is not very good.